DESCRIPTION OF PRIOR ART
Molding of thermosetting plastics has become widespread. Particularly in the automotive industry where weight is a significant factor, molding of plastic parts for interior parts and exterior panels has become common. As set forth in a brochure MD 79-012 of General Motors entitled "Molded Coating Process and Specifications," exterior class "A" panels are formed of a composite of specialty polyester resins, thickeners, thermoplastic copolymers, mineral fillers, fiber reinforcements, catalysts, and release agents. These materials are commonly combined (with different variations) in a sheet form and are known as "SMC." In the known molding technique of Compression Molding, a piece of SMC of the appropriate size is placed in a mold made up of a fixed and movable die which define a mold cavity therebetween. The dies are nested in the sense that they have side or shear edges which closely interfit and act as a seal during molding.
The dies are forced closed by a press ram under high forces that cause pressures on the substrate up to about 1500 psi. As a practical matter, the initial pressure must be high enough to form the substrate into the shape of the mold cavity. Depending on the size and shape of the part to be formed, different pressures are required. This molding process is commonly carried out at 300.degree. F. but different temperatures, for example, 100.degree. to about 400.degree. F., may be used.
The molded products resulting from Compression Molding can have surface defects, such as surface pits, porosity, sink marks and shrink cracks. In order to cover these defects to produce a class "A" surface, a technique of in-mold coating has been developed. The process for molding and in-mold coating presently includes the following sequence:
(1) the mold is opened and an SMC charge is placed between the dies in the mold cavity; PA1 (2) a press ram closes the mold under enough pressure to form or mold the substrate; PA1 (3) level control cylinders try to maintain the dies in parallel condition while closing; PA1 (4) after at least partial curing, the press ram releases the pressure; PA1 (5) cylinders force the mold open; PA1 (6) an injector shut-off pin for the coating opens; PA1 (7) the metering pump(s) stroke, injecting the coating; PA1 (8) the shut-off pin closes; PA1 (9) the press ram re-pressurizes; PA1 (10) the level control cylinder readjusts to try to maintain parallelism while reclosing to allow the coating to spread evenly across the part surface; PA1 (11) the coating is cured; and PA1 (12) the press ram is released, the dies are separated, and the part is removed.
The opening and closing of the mold, the depressurizing and repressurizing, and their accompanying level adjustment are major time delays in the in-mold coating process. A typical example of this process is illustrated in FIGS. 4 and 5 of U.S. Pat. No. 4,076,778 to Edwin D. Ditto, which is incorporated herein by reference. It is estimated that this opening and closing of the molds take up 5 to 20 percent of the total molding and coating process time. When the millions and millions of parts that are molded yearly are considered, the inefficiency and cost of this opening-closing routine are staggering.
The reopening and reclosing of the mold cause other problems as well. Opening of the dies causes the flashing around the shear edges to enter the mold cavity. Disturbance of the shear edge may cause an incomplete seal after reclosing. Moreover, the open-close coating method precludes the possibility of using secondary cores or slides in the mold. The slide usually comes in at an angle to one of the dies, and thus cannot be used if the die moves relative to the substrate. Another major limitation of in-mold coating as presently practiced is that it is not readily applicable to injection or reaction injection molding processes. These types of molding processes do not use dies that normally have shear edge construction.
It would be highly desirable to eliminate the secondary opening and closing of the dies from an efficiency and flexibility point of view.